Cellulose is the most abundant natural polymer. All forms of plant life contain cellulose. Because of its nearly ubiquitous distribution in nature, and human kinds' long exposure to cellulose, cellulose and its derivatives are generally recognized as the safest and most acceptable polymer class for use in food and pharmaceutical products. In its naturally occurring form, cellulose exists as a fibrous structure composed of arrays of long chains of cellulose molecules held together by van der Waal forces and interchain hydrogen bonds. The chemical structure of cellulose consists of repeating units of .beta.-D glucopyranose rings linked together by .beta.-1,4-glycosidic linkages. Depending on the degrees of order of arrangement and hydrogen bonding between cellulose chains, the crystallinity of the cellulose may range from 50% to 90%. The crystallinity of native cellulose is about 70% (P. H. Hermans and A. Weidinger, J. Poly. Sci., IV, 135,(1949)). The amorphous regions in the structure can result from damage during processing of pulp, from different chain bonding order (i.e., occurrence of .beta.-1,6-linkage instead of the regular .beta.-1,4-glycosidic bond) or as a result of natural imperfections. The degree of polymerization of cellulose may range from 1,000 to 10,000, depending on its source.
The reactions of cellulose with mineral acids to prepare non-fibrous, low molecular weight (i.e., low degree of polymerization) cellulose products suitable for use in food, cosmetics, pharmaceutical, and like products, have been extensively studied. The reactivity of cellulose towards acids depends on the crystallinity of the cellulose source, acid concentration, and the reaction temperature and duration. Several products with varying degrees of crystallinity and polymerization have been prepared. Battista (U.S. Pat. Nos. 2,978,446 and 3,146,170) disclose the preparation of level-off cellulose products suitable for the manufacture of microcrystalline cellulose--the most commonly and widely used direct compression excipient for pharmaceutical solid dosage form design, by reacting a cellulose material with 2.5N hydrochloric acid at boiling temperature for 15 minutes. According to the invention, the products produced are highly crystalline in nature. The level-off degree of polymerization values of products prepared from native fibers range between 200 and 300, whereas those prepared from regenerated cellulose lie in the range of from 15 to 60, and products prepared from alkali swollen natural forms of cellulose are in a degree of polymerization range between 60 and 125. Similar manufacturing procedures, to that described above, are described in German Patent DAS 1,123,460, using viscose cellulose as the starting cellulose source, and in Austrian Pat. No. 288,805. The use of gaseous HCl, at temperatures below 40.degree. C., without solvent, to prepare the cellulose precursor for microcrystalline cellulose, is disclosed in (East) German patent DD 71,282.
Ellefsen et al., in Norsk Skog Industri, 1959, p. 411, describe the preparation of crystalline cellulose products by dissolving the starting cellulose source in 38-40.3% concentrated hydrochloric acid at 20.degree. C., followed by precipitating with water. In U.S. Pat. No. 4,357,467, a similar procedure to the foregoing, using 37-42% HCl acid at 30.degree.-50.degree. C., is employed to prepare cellulose products having substantially reduced crystallinity (17-83%), and a low degree of polymerization (10-200). Compared to native and regenerated cellulose, the low crystallinity cellulose products show improved dispersibility in water, increased compatibility with basic compounds such as starches, proteins, and lipids, and are useful as excipients in the preparation of tablets and confectionery products.
Greidinger et al (U.S. Pat. No. 3,397,198) disclose the preparation of an amorphous degraded cellulose by treating a cellulose material with 65-75% sulfuric acid at a temperature of 35.degree.-45.degree. C. for a period of no longer than 10 minutes. The amorphous product is suitable for use in cleaners, cosmetic preparations, foodstuffs or as a filler for materials such as plaster-of-paris or adsorbents.
V. M. Brylyakove (SU Patent 4266981) describe the preparation of microcrystalline cellulose utilizing 3-5% nitric acid, sulfuric acid or hydrochloric acid and a fatty acid (C.sub.10-20) at 96.degree.-98.degree. C. The fatty acid enhances the efficiency of the process.
Other references that can be cited, pertinent to the preparation of microcrystalline cellulose, are: CA 111 (8) 59855w; CA 111 (8) 59787a; CA 108 (18) 152420y; CA 104 (22) 188512m; CA 104 (24) 209374K; CA 104 (24) 193881C; CA 99 (24) 196859y; CA 98 (12) 95486y; CA 94 (9) 64084d; and CA 85 (8) 48557u.
The interaction of cellulose with phosphoric acid has been the subject of several publications. S. M. Hudson and J. A. Cuculo, Macromol. Sci. -Rev. Macromol. Chem., C18, 6-7 (1980) and J. O. Warwicker, in Cellulose and Cellulose Derivatives," N. M. Bikales and L. Segal, eds., Wiley, New York, N.Y. (1971), Vol. V, Part IV, p. 325-79, describe that the swelling and/or dissolution of cellulose in phosphoric acid depend(s) on the concentration of the acid. In concentration range between 71-80%, the swelling of cellulose is rapid. Further increases in the concentration causes dissolution of the cellulose. According to Hudson and Cuculo, the dissolution of cellulose is incomplete when the acid solution contains higher than 85% and less than 92% phosphoric acid. S. N. Danilov and N. F. Gintse, Zh. Obsch. Khim., 26, 3014 (1956), describe that the cellulose dissolves more readily with increasing temperature, with a maximum dissolution rate at 40.degree.-50.degree. C.
Bellamy and Holub (U.S. Pat. No. 4,058,411) disclose the use of 80-85% phosphoric acid for the decrystallization of cellulose. According to the invention, the starting cellulose source, having particles about one millimeter in length and diameter, is reacted with phosphoric acid at room temperature, with or without the presence of a surfactant, for a prolonged period until a gel is formed. The product is then precipitated from the gel using an aqueous solution of tetrahydrofuran. The amorphous product can be used as a source of glucose or as a substrate for microbial production of antibiotics and other metabolites, single cell proteins and industrial alcohol.
In Swiss Pat. No. 79,809, a method is described for the depolymerization of cellulose using a mixture of hydrochloric acid and sulfuric acid or phosphoric acid (25-35%) at temperatures below 50.degree. C., is provided. There is, however, no mention of the crystallinity of the product in the disclosure.
We have found that the treatment of cellulose with phosphoric acid, under controlled sequenced temperature conditions, provides a rapid method of preparing low crystallinity cellulose products that are suitable for use as excipients in cosmetic, food, pharmaceutical, and like products.
Accordingly, the primary objective of this invention is to provide a rapid method for converting fibrous cellulose material to useful low crystallinity cellulose excipients using phosphoric acid.
A further objective of the present invention is to provide new low crystallinity cellulose excipients suitable for use in cosmetic, pharmaceutical, personal care, and like products.
Still another objective of this invention is to provide a bodying agent and/or film forming agent composed of hydrated low crystallinity cellulose.
These and other objectives of the present invention will be more apparent from the discussion that follows.